The present invention relates to surface-emission laser diodes and surface-emission laser arrays, optical interconnection systems, optical communication systems, electro-photographic systems, and optical disk systems.
In recent years, intensive studies are made with regard to surface-emission laser devices (surface-emission laser diodes) that produce a laser beam in the direction perpendicular to the substrate surface. As compared with an edge-emission laser diode, a surface-emission laser diode has a characteristically a small active layer volume, and hence, low threshold current for laser oscillation. Further, a surface-emission laser diode has an advantageous feature of the cavity structure suitable for high-speed modulation and can produce high-quality laser beam having a circular beam cross-section. Thus, a surface-emission laser diode attracts much attention in relation to the optical source of high-speed communication systems such as LAN or in relation to the optical source of electrophotographic systems.
Further, a surface-emission laser diode, emitting a laser beam in the direction perpendicularly to the substrate, can be easily integrated in the form of high-density two-dimensional array, and application of such a surface-emission laser array to the optical source of parallel optical interconnection systems, high-speed and high-resolution electrophotographic systems, and the like, is studied.
Currently, there are two major structures in the surface-emission laser diode according to the construction of the current confinement structure used for confining the injected current, the one being the surface-emission laser diode of selective oxidation type, and the other being the surface-emission laser diode of hydrogen ion implantation type. Any of these structures realizes significant decrease of oscillation threshold current by confining the current injection region to a specific region at the central part of the device.
The surface-emission laser diode of selective oxidation type achieves the current confinement by using an oxide layer of Al formed by selective oxidation of a semiconductor layer containing Al and is capable of achieving optical confinement of transverse mode in addition to the current confinement. Thereby, the laser diode can achieve the advantageous feature of low threshold current and high efficiency of operation.
In the case of the surface-emission laser diode of hydrogen ion implantation type, the current confinement structure is realized by ion implantation of hydrogen ions in the form of high-resistance region. With this, the surface-emission laser diode of hydrogen ion implantation type also achieves low threshold operation, similarly to the case of the laser diode of the selective oxidation type.
In any of these laser diodes, it should be noted that the feature of low-threshold current of laser oscillation is achieved by restricting the region of current injection to a particular area of the device located at a central part of the device structure.
For example, the Non-Patent Reference 1 discloses a surface-emission laser diode having an active layer of InGaAs and oscillating at the wavelength band of 0.98 μm. In this surface-emission laser diode, there is formed a selectively oxidized layer of Al0.98Ga0.02As in the upper Bragg reflector of the p-Al0.9Ga0.1As/GaAs structure formed above the active layer.
Such a surface-emission laser diode is produced by the steps of: etching the upper distributed Bragg reflector, after the crystal growth process thereof, to form a square mesa structure in such a manner that the sidewall surface of the Al0.98Ga0.02As layer to be oxidized is exposed; and forming the selectively oxidized layer by applying a selective oxidizing process to the foregoing Al0.98Ga0.02As layer starting from the mesa sidewall surface toward a mesa central region at the temperature of 425° C. in a water vapor ambient produced by bubbling water heated to 85° C. by a nitrogen gas.
As a result of the foregoing selective oxidation process, there is formed an insulation region of AlOx (oxide of Al) around the mesa structure, and associated with this, there is formed a conductive region at the central part of the mesa structure in the form of a non-oxidized region.
Thus, the selectively oxidized region of a surface-emission laser diode is generally formed by exposing a part of the AlGaAs selectively oxidizing layer to an ambient containing water vapor by an etching process, or the like.
With the surface-emission laser diode of this type, the holes supplied from the surface of the upper distributed Bragg reflector is injected into the active layer with confinement into the non-oxidized conductive region at the central part of the mesa structure. It should be noted that AlOx is an excellent insulator and can effectively restrict the region of hole injection to the active layer to the foregoing central part of the mesa structure. By using such a selectively oxidized structure, it becomes possible to reduce the oscillation threshold current drastically. In the Non-Patent Reference 2, a very low threshold current of 900 μA is achieved in the device having the non-oxidized region of 4.5 μm×8 μm.
Further, because of the fact that AlOx has a low refractive index of about 1.6, which is lower than the refractive index of other semiconductor layers, the surface-emission laser diode of the selective-oxidation type has an advantageous feature in that the optical beam formed as a result of laser oscillation is confined at the central part of the mesa structure as a result of formation of the lateral refractive index profile within the laser cavity structure. Thereby, optical loss caused by diffraction is reduced and the efficiency of the laser diode is improved.
On the other hand, associated with the increased degree of optical confinement, it becomes necessary to decrease diameter of the oxidized confinement region with such a laser diode for suppressing higher transverse mode oscillation. While it depends on the wavelength band, it is known that a single fundamental mode oscillation can be achieved with a surface-emission laser diode having a single oxidized current confinement structure, by reducing the diameter or edge length of the oxidized confinement structure down to 3-4 times as large as the oscillation wavelength. Thus, by using such a selective oxidizing structure, it becomes possible to achieve decrease of laser oscillation threshold and decrease of diffraction loss, in addition to the single fundamental mode control.
In the case of the surface-emission laser diode of the hydrogen ion implantation type, on the other hand, no built-in waveguide structure is provided contrary to the case of the surface-emission laser diode of the selective oxidation type.
In a surface-emission laser diode of the hydrogen ion implantation type, a waveguide structure is inducted by a refractive index change that is caused at the time of operation of the laser diode by the heat flowing through the device (thermal lens effect), and confinement of the transverse mode is achieved by using such a waveguide structure induced by the thermal lens effect.
Because the optical confinement obtained with such a laser diode is generally weak, it is possible with the surface-emission laser diode of the hydrogen ion implantation type, to obtain a single fundamental transverse mode oscillation even in the case the diameter of current confinement is relatively large. In the Non-Patent Reference 3, there is disclosed a surface-emission laser diode of the hydrogen ion implantation type using GaAs for the active layer and operating at the wavelength band of 0.85 μm. With the laser diode of the Non-Patent Reference 3, an oscillation threshold current of 2.5 mA is obtained by using a hole injection region having a diameter of 10 μm.
In many applications of surface-emission laser diode, there exists a request, in addition to the request for low threshold characteristics, in that the laser diode provides a single peak beam shape at high output power state. Thus, control of single transverse mode is a very important object in the surface-emission laser diode. Generally, in surface-emission laser diode, control of the single transverse mode is possible only in the state in which the laser diode is operating at a relatively low current injection level. When the current injection level is increased, there is caused oscillation of higher order transverse mode due to the spatial hole-burning effect of carriers.
More specifically, when the laser diode is operating under the high current injection state, there occurs an increase in the photon density inside the optical cavity, while such an increase of the photon density facilitates increase of stimulated emission in the part where the optical intensity is large. As a result of such an increased stimulated emission, there is caused a localized dip of carrier density (spatial hole-burning phenomenon).
Because the fundamental transverse mode has a large mode amplitude (electric field distribution) at the central part of the mesa structure, there occurs a decrease of carrier density at such a central part of the mesa structure with increase of the injection current, while this decrease of the carrier density at the mesa central part leads to saturation of laser gain for the fundamental transverse mode. On the other hand, at the peripheral part of the mesa structure surrounding the mesa central part, there occurs an increase of carrier density, and with this, a laser oscillation is caused for the higher-order transverse mode having a mode amplitude in the region between the mesa central part and the mesa peripheral part because of increase of laser gain in such a part of the laser diode.
It should be noted that this phenomenon appears particularly conspicuous in the selective-oxidation type surface emission laser diode in which there is caused a strong mode confinement by using the selective oxidized structure. Thereby, the quality of the exiting laser beam is deteriorated seriously.
In the case of the surface-emission laser diode of the hydrogen ion implantation type, there is provided no such built-in optical confinement structure, and because of this, the laser diode shows poor stability for the transverse mode. Thus, the laser diode easily causes higher mode oscillation when the injection current is increased.
In order to suppress the laser oscillation of higher order transverse mode in such a surface-emission laser diode, various proposals have been made so far.
For example, the Non-Patent Reference 4 discloses an approach of suppressing the higher order transverse mode oscillation by forming an antiguiding structure in a part of the cavity structure, by selectively oxidizing a layer of Al0.9Ga0.1As from the crystal growth surface in correspondence to the current injection region at the central part of the mesa structure.
In this example, there is formed an antiguiding structure, in a surface-emission laser diode oscillating at the 0.98 μm wavelength and having a current confinement structure of the diameter of 10-17 μm, the current confinement structure being the one formed by selective oxidation of the AlAs selective oxidizing layer having the thickness of 18.6 nm in a high-temperature water vapor ambient, starting from the mesa etching sidewall surface, by selectively oxidizing a mixed crystal of Al0.9Ga0.1As having a thickness of ½λ, leaving the current injection region at the central part of the mesa structure over the region of 15 μm in diameter. With this, the higher order transverse mode oscillation is effectively suppressed and a single peak radiation pattern is obtained even in the case a drive current twenty times as large as the threshold current is injected.
In the example of the Non-Patent Reference 4, the spatial overlapping of the higher order transverse mode and the gain region in the active layer (current injection region) is decreased by providing the antiguiding structure at the central part of the cavity structure. With this, it becomes possible to suppress the laser oscillation at the higher transverse mode.
FIGS. 3A and 3B are diagrams for explaining the effect of the antiguiding structure provided partially in the optical cavity on the electric field distribution of the fundamental mode and the first-order higher mode. Here, it should be noted that FIG. 3A is a diagram showing the mode distribution for the case the anti-guiding structure is not provided, while FIG. 3B shows the mode distribution for the case an antiguiding structure is provided at the central part of the device. In FIGS. 3A and 3B, the upper diagram shows the fundamental transverse mode distribution, while the lower diagram shows the first-order transverse mode distribution. Further, the gain region formed with the current injection is also represented in FIGS. 3A and 3B.
As can be seen from FIG. 3A, it is difficult to spread the mode distribution, in the case of the fundamental transverse mode having a large mode distribution at the central part of the mesa structure, toward the peripheral direction of the mesa structure by the antiguiding structure, while in the case of the higher order transverse mode, the electric field strength is zero at the center of the mesa structure and there appears a large mode distribution at the peripheral part of the mesa structure as shown in FIG. 3B. Thus, it becomes possible with the antiguiding structure to deform and expand the mode distribution profile laterally toward the mesa peripheral direction, and it becomes possible to decrease the mode distribution (electric field strength) at the mesa central part.
Thus, from the reason explained above, the surface-emission laser diode can maintain the fundamental transverse mode operation up to high output state with such an antiguiding structure, which decreases the gain for the higher order transverse modes. Further, with regard to the fundamental transverse mode, it is difficult to modify and expand the mode distribution profile in the lateral direction by the antiguiding structure, although it is also expected that the use of such an antiguiding structure results in somewhat broadening of the electric field distribution at the mesa central part. Thereby, the electric field intensity at the mesa central part is decreased, and it is expected that occurrence of the spatial hole-burning phenomenon is suppressed.
Meanwhile, in the case of a surface-emission laser diode for use as the optical source in the applications other than the optical communication technology, there is a strong demand, in addition to the demand for the circular beam shape suitable for optical coupling with optical fibers, in that the laser diode provides a single fundamental mode oscillation, while this requirement can be met, in the case of the surface-emission laser diode of the selective oxidation type, by setting the diameter of the current confinement structure to be less than about three times as large as the oscillation wavelength. In the case of the single-mode device, on the other hand, increase of device resistance and electrical capacitance associated with the selective oxidation structure imposes a limitation upon the modulation frequency band. Further, spatial hole-burning effect raises the problem that it is difficult to achieve high-power laser oscillation in the single fundamental mode.
In the case of the surface-emission laser diode of the hydrogen ion implantation type, on the other hand, there occurs no problem of parasitic capacitance as in the case of the current confinement structure formed by the selective oxidation process. In the surface-emission laser diode of the hydrogen ion implantation type, there exists no built-in waveguide structure, and confinement of transverse mode is achieved by the small refractive index change caused by heat generation, which in turn is caused primarily by the current injection. Thus, in the case of the surface-emission laser diode of the hydrogen ion implantation type, the degree of lateral confinement in the device is inherently weak, while this leads to the disadvantageous feature of unstable transverse mode control associated with the weak lateral confinement, although there is obtained an advantageous feature that it is possible to achieve single fundamental transverse mode oscillation for the case of relatively large confinement diameter.
Non-Patent Reference 5 discloses a surface-emission laser diode capable of maintaining the single fundamental transverse mode oscillation under the state of laser oscillation with high output power, called ARROW (antiresonant reflecting optical waveguides) structure or S-ARROW (Simplified ARROW) structure.
In Non-Patent Reference 5, it should be noted that the object of providing a surface-emission laser diode structure capable of maintaining the single fundamental transverse mode oscillation under the state of high laser oscillation power is attempted by way of using an antiguiding structure. Here, it should be noted that an antiguiding structure is a waveguide structure in which there is provided a low refractive index core part in correspondence to the oscillation region of the laser diode in such a manner that the low refractive index core part is sandwiched by regions of relatively large refractive index in the direction of the laser cavity. With such an antiguiding structure, leakage of higher-order transverse mode is facilitated in the direction perpendicular to the laser cavity direction, and with this, it becomes possible to maintain the single fundamental transverse mode oscillation up to the state of high output power.
Further, Non-Patent Reference 6 teaches an antiguiding structure in which there is provided a periodic structure formed of a low refractive index region and a high refractive index region adjacent to the oscillation region of the device constituting the low refractive index core such that there is formed a cavity structure in which the low refractive index core forms a half-wavelength resonator.
In this Non-Patent Reference 6, leakage of the transverse mode pertinent to the antiguiding structure is reduced, and with this, a single fundamental transverse mode oscillation is achieved with the power exceeding 7 mW.
Meanwhile, in the application of the surface-emission laser diode to the long-range and super-fast optical communications, there arises a problem, in addition to the single fundamental transverse mode oscillation, of noise caused by the change of reflectivity of the optical components used therein depending on the polarization direction of the laser beam. Thus, there is a demand that the laser diode produces the laser beam with polarization controlled to a specific polarization direction.
Conventionally, there has been disclosed a method of controlling the polarization direction of laser beam by using a strained film as disclosed in the Patent Reference 5, in which anisotropic stress is applied to the laser diode or anisotropy is caused in the gain of the active layer. Further, the Patent Reference 6 discloses a method of forming the surface-emission laser diode on an inclined substrate. In any of these methods, there is induced an anisotropy in the gain caused in the active layer, and it becomes possible to control the polarization direction in the direction of large optical gain.
(Non-Patent Reference 1)
Applied Physics Letters vol. 66, No. 25 pp. 3413-3415, 1995 and Electronics Letters No. 24, Vol. 30, pp. 2043-2044, 1994
(Non-Patent Reference 2)
Electronics Letters No. 24, Vol. 30, pp. 2043-2044, 1994
(Non-Patent Reference 3)
Electronics Letters No. 5, Vol. 27, 1991, pp. 457-458
(Non-Patent Reference 4)
IEEE Photonics Technology Letters Vol. 10, No. 1, 1998, pp. 12-14
(Non-Patent Reference 5)
Applied Physics Letters vol. 76, No. 13, 2000, pp. 1659; IEEE Journal of Quantum Electronics vol. 38, No. 12, 2002, pp. 1599.
(Non-Patent Reference 6)
IEEE Journal of Quantum Electronics, vol. 38, No. 12, pp. 1599.
(Patent Reference 1)
Japanese Laid-Open Patent Application 11-54838
(Patent Reference 2)
Japanese Laid-Open Patent Application 20001-60739
In the surface-emission laser diode according to the Non-Patent Reference 4 cited before, it should be noted that the antiguiding structure of AlOx is provided inside the cavity. Thus, in order to form such an antiguiding structure, there is conducted a selective oxidation process of an AlGaAs layer having a large Al content in a high-temperature water vapor environment. This process will be referred to hereinafter as water vapor selective oxidation.
Now, in the case of forming such an antiguiding structure, it is necessary to provide the AlOx layer constituting the antiguiding structure at the central part of the cavity structure where the fundamental mode amplitude is large. This means that it is not possible to apply a conventional method of oxidizing the selectively oxidizing layer starting from the sidewall surface of the mesa structure as used in the conventional surface-emission laser diode, and the selective oxidation has to be conducted from the device surface.
Further, in order to secure sufficient effect of the antiguiding structure for the higher mode oscillation, it is necessary to provide the anti-guiding structure in correspondence to the part of the cavity where the electric field strength is relatively large. This means that the antiguiding structure has to be formed inside the upper distributed Bragg reflector.
However, epitaxial growth of a semiconductor layer is not possible on an AlOx layer, and thus, it is necessary with such a surface-emission laser diode to provide a dielectric distributed Bragg reflector on such an AlOx layer to form a laser structure. However, such a laser structure has a drawback in that it requires not only an additional process of evaporation deposition of the dielectric layers but also an etching process for removing a part of the distributed Bragg reflector so as to enable injection of drive current via a top electrode.
Further, because there is a need of providing the selective oxidizing layer used for forming the antiguiding structure always at the surface part of the device structure with the laser diode of such a structure in anticipation of the selective oxidation processing conducted in the high temperature ambient containing water vapor, there is imposed a restriction on the degree of freedom of designing the device.
Further, because a dielectric multilayer mirror is provided in the upper part of the laser diode with the surface-emission laser diode of this type, the distance between the p-side electrode and the selective oxidation layer is reduced, and thus, the holes are injected into the confinement region in the lateral direction from the peripheral part of the AlOx layer constituting the antiguiding structure. Thereby, the device resistance of the laser diode is inevitably increased.
Further, as noted before, it is necessary that the polarization direction of the output laser beam is controlled to a specific direction in the case the laser diode is to be used for the optical fiber communications or for the optical source of electro-photographic systems.
In the application of the surface-emission laser diode to high-speed optical fiber communications, there arises a problem of noise caused by the polarization-dependent difference of reflection or transmission characteristics of the optical components constituting the system. Further, in the application of optical writing, too, there arises a problem with the polarization dependence of the optical systems used therein that an optical beam spot is distorted on the surface of the photosensitive body.
Because the foregoing reference does not describe the control of polarization direction, it is difficult to use the teaching therein directly for the laser diode device used for actual applications.
With regard to the attempt of controlling the polarization direction, it is proposed to form the laser diode on an inclined substrate or apply an anisotropic stress to the device as noted before. However, the former approach has a drawback in that the range of crystal growth condition used at the time of the crystal growth process of the laser diode structure is limited. Further, in the latter approach, controllability of polarization depends heavily on the controllability and reproducibility of processing condition.
Further, with the method of forming a surface-emission laser diode on an inclined substrate as described in the foregoing Patent Reference 2, there is a problem that adjustment and control of crystal growth condition is difficult ion such an inclined substrate as compared with the substrate having the (100) surface orientation. Further, while it is possible to control the polarization direction of the laser oscillation beam in a particular direction along a specific crystal orientation with such a method, it is difficult to control the polarization direction in an arbitrary direction for individual devices.
Further, with the method of applying an anisotropic strain to the active layer by using the thin film accumulating therein a stress as set forth in Patent Reference 1, there arises a problem that the control of polarization becomes unstable because of the reproducibility and controllability of the processing condition.